Patent expiration opens the door to generics, a fate some popular medications will face over the next few years. The result is sharply lower prices for patients and lower sales for drug makers.

For millions of Americans, prescription drugs are about to get a lot cheaper.

Patents on some of the most popular medications will expire over the next few years, giving consumers access to less expensive generic versions — and costing the pharmaceutical industry an estimated $100 billion in lost sales through 2015.

Lipitor, a cholesterol-fighting medication that is the top-selling prescription drug of all time, lost its patent protection Wednesday. The drug's manufacturer, Pfizer Inc., already has slashed its price to as little as $4 a month for privately insured patients, the majority of Lipitor users. That's down from typical co-pays of $25 to $45.

In the months to come, patents will expire on other popular drugs including Lexapro, used to treat depression, and Plavix, which is widely prescribed for blood thinning. Asthma sufferers will be able to get generic versions of Singulair next summer.

Experts say the timing is largely coincidental but will pay big benefits: Prices typically fall as much as 90% when generics come on the market.

"It's great news for patients because they are going to experience savings that I call the patent dividend," said Michael Kleinrock, director of research development at the IMS Institute for Healthcare Informatics, which tracks the pharmaceutical industry.

Wisconsin retiree Gloria Schmid, 68, said the lower price would help her afford her Lipitor prescription on a tight budget. She has been paying $63 a month for the medicine even with insurance.

"That would be marvelous," Schmid said of the savings. "As you get older, your resources get to be less and less. That's what worries me."

But the patent expirations pose a challenge for the drug companies, which have come to expect large payoffs for their patented medicines.

IMS estimates drug companies will lose $100 billion in revenue over the next four years, and that will diminish the money available to develop the next generation of popular drugs. Generics make up 79% of all prescription medications dispensed in the U.S.

In addition, much of the pioneering pharmaceutical work underway is for treating diseases that afflict fewer people, such as certain cancers, and probably won't lead to drugs with huge sales figures.

"The era of blockbuster drugs is gone," said Alexander Kandybin, a partner and health specialist at Booz & Co., a management consulting firm in New York. "The pipeline will not replace that revenue. It's inherently unpredictable how the industry is going to evolve over the next five years."

Drug makers also are encountering tougher regulatory scrutiny, growing competition from manufacturers in the U.S. and overseas and resistance from cost-conscious health insurers.

To counter these forces, drug companies are fighting to protect the markets they already have.

Pfizer has rung up an estimated $80 billion in sales of Lipitor and has launched an aggressive campaign against generic competition.

The New York drug maker is offering sharp discounts on Lipitor through pharmacy benefit companies, as well as producing an "authorized" generic version and charging just $4 monthly co-pays for privately insured customers who get discount cards to purchase the brand-name pills.

Right now there are just two other players. For the next six months, Ranbaxy Laboratories of India and Watson Pharmaceuticals of New Jersey get to market generic versions of the drug. Lipitor prices for those without insurance are not expected to drop sharply until next June, when other drug makers can bring their versions to market.

A Pfizer spokesman said consumers can expect to buy brand-name Lipitor for less than they pay for a generic version.

"Our strategy during the 180-day period is to help patients who want to stay on Lipitor have access to the brand after loss of exclusivity," spokesman MacKay Jimeson said. "Our programs, which are designed to offer Lipitor at or below generic cost during the 180-day period, will not increase costs for the significant number of payers participating in our programs."

Analysts have suggested that Pfizer could retain as much as 40% of its Lipitor volume in the coming months. But revenue will eventually plunge in the same way the price of other brand-name drugs fall when they lose their patents.

Other prescription drugs facing patent expirations next year are Actos, Seroquel, Diovan and Geodon.

"It's going from competing for dollars to competing for pennies," said Joel Hay, a professor of pharmaceutical economics and policy at USC. "That's got the brand-name industry in a quandary."

As generics come to market, patients' savings will vary based on several factors, including whether they have health insurance and how they obtain their medicine.

The uninsured will also benefit. Patients who take the coronary drug Plavix, for example, now pay an average of $172 a month at the pharmacy, according to the IMS healthcare institute. Generic versions could cut that bill to less than $20 if the past is any guide.